Generally speaking a good to great team shoots for ratings in the core areas of 60+ at DIII, 65+ at DII, 70+ at low DI, 75+ at mid-major, and 80+ at the higher levels of DI (and really, as high as you can get if you want a national title contending DI team). That's my understanding.
Although I will say that a team can easily make the postseason and win games with ratings that are less than those numbers. I've even had teams make deep runs in the NT with cores slightly below those I mention.